The present invention relates to a hydraulic control system mounted on construction machines such as hydraulic excavators and cranes, and more particularly to a hydraulic control system equipped with a recovery circuit for recovering a return fluid from a hydraulic actuator to the supply side when the pressure of a hydraulic fluid supplied to the actuator is small.
One of prior art hydraulic control systems equipped with recovery circuits is disclosed in JP, B, 4-59484, for example. This known hydraulic control system comprises a hydraulic pump, a hydraulic actuator, e.g., a hydraulic cylinder, driven by a hydraulic fluid delivered from the hydraulic pump, a directional control valve for controlling a flow of the hydraulic fluid supplied from the hydraulic pump to the hydraulic cylinder, and a recovery circuit for recovering a return fluid from the hydraulic cylinder to the supply side of the hydraulic cylinder when the pressure of the hydraulic fluid supplied to the hydraulic cylinder is small.
The recovery circuit comprises a recovery passage communicating between a drain passage and a supply passage in the directional control valve, a check valve provided in the recovery passage for allowing the hydraulic fluid to flow only in a direction from the drain passage to the supply passage, a recovery switching valve provided in the drain passage in the directional control valve, a pressure detecting passage for detecting a pressure in the supply passage in the directional control valve and transmitting the detected pressure to the recovery switching valve, and a pressure signal generator provided outside the directional control valve for generating a set pressure Pc for the recovery switching valve.
When the directional control valve is operated in a direction to extend the hydraulic cylinder, the hydraulic fluid delivered from the hydraulic pump flows through the supply passage in the directional control valve and then enters a bottom-side chamber of the hydraulic cylinder. The hydraulic fluid flowing out of a rod-side chamber of the hydraulic fluid is returned to a reservoir through the drain passage in the directional control valve. In addition, the pressure in the supply passage in the directional control valve is detected through the pressure detecting passage. When the detected pressure is lower than the set pressure Pc from the pressure signal generator, the drain passage is closed by the recovery switching valve in the directional control valve to effect a recovery function so that all of the return fluid from the rod-side chamber of the hydraulic cylinder is additively introduced to the supply passage through the recovery passage and the check valve in the directional control valve. When the load of the hydraulic cylinder is increased and the pressure in the supply passage is raised to such an extent that the detected pressure becomes higher than the set pressure Pc from the pressure signal generator, the drain passage closed by the recovery switching valve is now communicated with the reservoir. Therefore, the recovery function is ceased and the return fluid from the rod-side chamber of the hydraulic cylinder is returned to the reservoir without being introduced to the supply passage. In other words, when the load pressure generated upon the hydraulic cylinder being extended is lower than the set value, all of the return fluid from the rod-side chamber is recovered and introduced to the bottom-side chamber additively to increase the operating speed, but when the load pressure is increased, the hydraulic fluid is supplied only at the delivery rate of the hydraulic pump to the bottom-side chamber of the hydraulic cylinder.